Filament for evaporating reactive metal in high vacuum apparatus



Jan. 25, 1966 HALL 3,231,715

FILAMENT FOR EVAPORATING REACTIVE METAL IN HIGH VACUUM APPARATUS FiledMarch 18, 1965 m 2' J u. 2: 8

a INVENTOR LEWIS D. HALL r- BY l ATTORN EYS United States Patent3,231,715 FILAMENT FOR EVAPORATING REACTIVE METAL IN HIGH VACUUMAPPARATUS Lewis D. Hall, Palo Alto, Calif, assignor to UltelrCorporation, Palo Alto, Calif., a corporation of Cahfornia Filed Mar.18, 1963, Ser. No. 265,709 6 Claims. (Cl. 219-275) This inventionrelates to high and ultrahigh vacuum equipment and more particularly toapparatus for evaporating or subliming chemically reactive metal to formdeposits which will trap or getter gas in a vacuum system.

In general, evaporator units include a filament and suitable holderassembly. The filament can include a reactive metal wire, such astitanium, closely wound around a refractory supporting core which iselectrically heated to evaporate the titanium.

Ideally, in evaporator units for practical commercial high vacuumapparatus, the amount of metal evaporated for each watt of electricalpower applied should be high to provide maximum efiiciency. Total wattsof input power should be low to minimize heating. As is known, thegene-ration of heat in vacuum systems causes outgassing of componentswhich tends to raise the pressure in the vacuum system. In dynamic,commercial equipment this problem has dictated the practice of employingelaborate cooling schemes, not uncommonly employing liquid nitrogen andthe like. Where continuous pumping for extended periods in dynamicsystems with high gas loads is desired, as much reactive metal aspossible should be supplied without unduly increasing total watts ofinput power.

The foregoing objectives are, therefore, mutually incompatible and incommercial equipment represent severe restrictions. Thus, the totalamount of reactive metal evaporated can be increased by enlarging thewire diameter of the reactive metal winding. However, the resultantincrease in the power requirement tends to dictate installation ofcostly liquid cooling. If wire size is kept sufficiently small whereby afan can provide simple air cooling, then the supply of reactive metal isinadequate for practical use.

Furthermore, in evaporator filaments of the kind described above, thereactive metal must be carefully heated to evaporation temperaturewithout melting the metal. Using titanium, for example, once it beginsto melt, the entire supply promptly melts and runs freely from the core.

It is a general object of the invention to provide an improvedevaporator unit.

Another object of the invention is to provide an improved filament forevaporation or sublimation of reactive metal.

A more particular object of the invention is to provide an evaporatorfilament combining enhanced efliciency of evaporation, a powerrequirement low enough to employ air cooling, and a supply of reactivemetal adequate for extended operation.

These and other objects of the invention will become more readilyapparent from the following detailed description of a preferredembodiment when taken in conjunction with the accompanying drawing, inwhich:

FIGURE 1 shows, partly in section, a holder for supporting an evaporatorfilament;

FIGURE 1A shows in side elevation, a filament for use in the holdershown in FIGURE 1;

FIGURE 2 is an enlarged section view of FIGURE 1A in the region of line22 thereof;

FIGURE 3 is an elevation section view further enlarged 2 of the detaildefined by line 3-3 of FIGURE 2; and

FIGURE 4 is similar to the detail of FIGURE 3 schematically showing thechange therein after heating.

Generally, the filament for evaporation or sublimation includes arefractory support member adapted to be resistance heated. Axiallyspaced dams of refractory material encircling the support member areprovided therealong. The interstices between the dams include .a mass ofreactive metal of a kind to be evaporated whereby the metal-filledinterstices and dams generally define an encircling thickness, hereinreferred to as a first layer. An overlying layer of reactive metal to beevaporated is then disposed to encircle the first layer. The dams areundercut to permit the mass of reactive metal in the first layer tospread when softened by heating thereby providing enlargement of theheat transfer interface between the ma-ss of reactive material and thesupport member. For example, the dams can be formed of a helically woundrefractory wire. Refractory co-planar rings can also be distributedalong the support member to form the dam-s. The completed filament iscarried into the vacuum system by a holder assembly whereby spentfilaments can be readily removed and replaced.

Referring to FIGURE 1A, a filament 10 is shown which is adapted to besupported between holders 21, 22 of refractory material, such asmolybdenum, carried in a holder assembly designated generally by thereference numeral 24) as shown in FIGURE 1.

The above first layer" of refractory dams and evaporative reactive metalis generally provided by a pair of closely wound, elongated wire orribbon members wrapped around the refractory support member to form afirst layer therealong. One of the ribbon members is comprisedessentially of a refractory matenial and the other a reactive metal. Asecond layer then is applied consisting essentially of reactive metalmost conveniently in the form of a closely wound reactive metal wire orribbon member.

It has been observed that with the foregoing filament arrangement, eachencircling turn of the refractory ribbon member serves so that thereactive metal in the first layer can melt without running freely fromthe filament. At the same time, there is provided enhanced heatconductance via the interface with the support member since the annularrefractory ribbon member permits the reactive metal, when softened byheat, to spread over an enlarged area of the support member due to itsnarrow circumferential line contact with the refractory ribbon member.In this manner, the transfer of heat from the support member ischanneled primarily through the metallic mass between turns of therefractory winding. Further, the second layer tends to blend with thespreading material of the first layer to provide a highly heatconductive path throughout the reactive material being evaporated.

It has also been observed that this arrangement serves to minimize thepower required to evaporate an overlying layer of reactive metal. Thepower required to evaporate the overlying layer is only very little morethan that required to evaporate a single layer. The requisite power forevaporation of the filament is mostly utilized to maintain heat ofevaporation in the outermost subsurface region of the reactive metal. Inother words, the rate of heat loss is greatest near the exposed surface,whereas the heat loss distribution curve along a path extending radiallythrough the rest of the reactive metal is otherwise fairly flat. Theaddition of the second layer, therefore, serves to permit the reactivematerial of the first layer to become hotter throughout its completethickness and thereby increase efiiciency of evaporation.

Referring to the drawings, filament 10 comprises a rodlike core member11 of resistance heatable refractory material. The material must besufficiently rigid whereby when heated, it can properly carry theover-windings mentioned above. Suitable core material can be selectedfrom the group consisting essentially of metals and alloys includingrhenrium, tungsten-rhenium, tungsten-molybdenum, or tungsten. The latterhas been found to be particularly satisfactory.

The first layer 13 is formed by a bifil-ar close wound windingcomprising a wire 14 of refractory material and another wire 15 ofreactive metal material of a kind to be evaporated. The matenial of wire14 can be selected from any suitable ductile, refractory material whichlends itself to being close wound about member 11. Suitable refractorymaterials for this purpose include metals and alloys of the groupconsisting essentially of niobium, tantalum and molybdenum. The latteris particularly satisfactory. The refractory material for wire 14 mustbe such as to raise the melting point of the reactive metal of wire 15when brought into con-tact with it. Furthermore, it must not form lowmelting eutectics with the reactive metal.

Suitable material for wire 15 must be capable of being bent around asmall radius and, therefore, cannot be too stiff. Suitable reactivematerial, therefore, may be selected from the group consistingessentially of vanadium, yttrium, or titanium, or alloys thereof.Titanium has been found to give particularly good results.

In order to provide layer 13 with a substantially uniform thickness, thediameter of wires 14, 15 preferably should be substantially the same.While these diameters can vary between limits depending upon the powerrequirment of the unit, a particularly desirable range of diameters hasbeen found to lie between on the order of 0.015 and 0.045 inch usingeither molybdenum o1 niobium.

An overlying layer 16 of reactive metal to be evaporated is preferablyformed by a close wound wire 17 of suitably ductile chemically reactivemetal preferably selected from the group consisting essentially ofvanadium, yttrium, titanium or alloys thereof. Particularly advantageousresults have been obtained using titanium. An adequate commerciallypracticable supply of reactive metal commensurate with a low powerrequirement is obtained using a wire diameter on the order of between0.015 inch to 0.045 inch.

Example 1 A particularly advantageous example of filament has beenformed by employing a tungsten core member 11 of 0.022 inch diameter anda molybdenum Wire 14 having a diameter of 0.015 inch and a titanium wire15 of substantially like diameter. Wire 17 of layer 16 is formed with adiameter of 0.015 inch of titanium.

Another example of a suitable filament can include a tungsten core 11having a diameter of 0.065 inch, a molybdenum wire 14 and titanium wire15 having a diameter of 0.045 inch and an overwinding wire 17 oftitanium having a diameter of 0.045 inch.

Yet another example includes a tungsten core 11 having a diameter of0.020 inch, a molybdenum wire 14 and titanium wire 15 each having adiameter of 0.010 inch and an overwinding wire 17 of titanium having adiameter of 0.020 inch.

Holder assembly 20 supports filament 10 whereby it can be heated toevaporate the metal of Wires 17 and 15.

Means for disposing filament 10 in a vacuum system include a baseassembly 23. Assembly 23 includes a flanged cylindrical base member 24.Member 24 carries a plurality of studs 25 attached thereto by screws. Acylindrical sleeve 26 is supported by base member 24 and closed at itsopen end by a top cap member 27. Cap member 27 is retained against theend of member 26 by screws carried in the ends of studs 25.

Means for supporting filament 10 in a heating circuit include a sinteredmetallic oxide terminal 28 such as sold by the U.S. Stoneware Co.,Tallmadge, Ohio, under the mark Alite. Terminal 28 extends throughmember 24 and connects with an electrode 29. A braided cable 30 and capconnector 31 form a lead-in to electrode 29. Cable 30 is disposed to beenergized by connections made with a terminal supported by theinsulating bushing 32. A refractory insert 22, for example, ofmolybdenum, is carried in the end of electrode 29 and adapted to receiveone end of filament 10.

Base member 24 supports a second electrode 33. Means whereby mutualsupport is provided between electrodes 29 and 33 includes an annularceramic bead 34 carried by electrode 29.. A support plate 35 is retainedon bead 34 by washers 36. Electrode 33 extends through an opening inplate 35. The free end of electrode 33 carries a filament-retainingmember 37 formed with an opening 38. Opening 38 extends through member37 and is aligned with holder 22. Opening 38 similarly carries arefractory holder 21, adapted to receive one end of filament 10. Aretaining collar 39, together with a high vacuum O-ring seal 40encircles the flange of base mem ber 24 whereby the entire unit can bescrewed into place and retained by a coacting fitting of the vacuumsystem (not shown).

Thus, filament 10 is disposed to be connected into an electrical circuitwhereby it can be heated to evaporate the reactive metal thereon.

In operation it has been observed that the reactive metal of wire 15will soften and tend to spread beneath the turns of refractory wire 14as in the region 41. This spreading has been observed to provideenhanced heat conductance between core member 11 and wire 15 byenlarging the interface therebetween. As wire 15 is heated and softens,heat transfer therethrough serves to soften the turns of wire 17 asshown in FIGURE 4 tending to provide a merged mass of reactive metalfree of heat retarding interfaces. As noted above, it has been observedthat the temperature distribution through wire 15, for example betweenpoints a and b on the axis 42 will have only a slight slope, whereas thegreatest heat loss can be expected to be found near the exposed surfaceof layer 16, for example, say between points 0 and d. On the other hand,heat transfer through the refractory wire 14 is impeded substantially bythe circumferential line contact at e. The turns of wire 14 form dams toprevent the heated mass of wire 15 therebetween from running freely fromcore 11.

From the foregoing it will be evident that considerably more titaniumcan be evaporated with a very small percentage increase in power. Forexample, using the structure described in Example 1 above, the powerrequirement to evaporate the titanium therein can be on the order of 250watts. However, on the of 240 watts are required if layer 16 is notemployed. Accordingly, virtually three times more titanium can beevaporated with only a 4% increase in power.

I claim:

1. A filament for evaporating reactive metal in a vacuum system to trapgases therein, comprising a support member of resistance heatablerefractory material, means forming axially spaced dams of refractorymaterial along said core member, a mass of reactive metal of a kind tobe evaporated disposed around said member between said dams to define afirst layer therewith, and a layer composed only of reactive metal to beevaporated extending along said support member overlying said firstlayer and in heat conducting physical contact with at least the reactivemetal of said first layer.

2. A filament as defined in claim 1 wherein the dams are undercut topermit said mass of reactive metal to spread thereunder, upon softening,to provide enlargement of the heat transfer interface between said massof reactive metal and said support member.

3. A filament as defined in claim 1 wherein the dams are formed byspaced turns of wire closely encircling said support member.

4. A filament for evaporating reactive metal in a vacuum system to trapgases therein comprising a support member of resistance heatablerefractory material, a pair of elongated ribbon members closely Woundaround said member to form a first layer thereon, one of said ribbonmembers being comprised essentially of a refractory material and theother of a reactive metal, and a second layer disposed contiguouslyaround said first layer and consisting essentially of only reactivemetal to be evaporated, said second layer being in heat conductingphysical contact with at least the reactive metal of said first layer.

5. A filament as defined in claim 4 wherein the refractory material ofsaid first ribbon member consists essentially of metals or alloysselected from the group consisting of molybdenum, niobium and tantalum.

6. A filament for evaporating reactive metal in a vacuum system to trapgases therein comprising a stilt tungsten core member, a pair ofWire-like ribbon members closely wound around said member to form afirst layer thereon, one of said ribbon members being comprisedessentially of molybdenum and the other ribbon member of titanium, and asecond layer contiguous to said first layer and consisting essentiallyof only titanium, said second layer being in heat conducting physicalcontact with at least the reactive metal of said first layer.

References Cited by the Examiner UNITED STATES PATENTS 2,336,13812/!1943 Van Hoorn et al 316-6 2,660,540 11/1953 Karash et al.

2,960,618 11/1960 Waer.-

2,967,012 l/ 1961 Connor.

2,975,075 3/1961 Beese ll 849 X 2,986,326 5/1961 Landfors 230693,068,337 12/1962 Kuebrich et al. 118-49 X 3,117,210 1/19 64 Herb 23069RICHARD M. WOOD, Primary Examiner.

1. A FILAMENT FOR EVAPORATING REACTIVE METAL IN A VACUUM SYSTEM TO TRAPGASES THEREIN, COMPRISING A SUPPORT MEMBER OF RESISTANCE HEATABLEREFRACTORY MATERIAL, MEANS FORMING AXIALLY SPACED DAMS OF REFRACTORYMATERIAL ALONG SAID CORE MEMBER, A MASS OF REACTIVE METAL OF A KIND TOBE EVAPORATED DISPOSED AROUND SAID MEMBER BETWEEN SAID DAMS TO DEFINE AFIRST LAYER THEREWITH, AND A LAYER COMPOSED ONLY OF REACTIVE METAL TO BEEVAPORATED EXTENDING ALONG SAID SUPPORT MEMBER OVERLYING SAID FIRSTLAYER AND IN HEAT CONDUCTING PHYSICAL CONTACT WITH AT LEAST THE REACTIVEMETAL OF SAID FIRST LAYER.